1. Field of the Invention
The present invention relates to an optical switch which switches paths by wavelengths.
2. Description of the Related Art
For accommodating vastly increasing internet traffic, optical networks, such as WDM (Wavelength Division Multiplexing) communication systems, are rapidly spreading. Although point-to-point networks are the mainstream of the present WDM, ring-type networks and mesh-type networks will be more developed in the near future, and at each node which forms a network, adding/dropping of arbitrary wavelengths and Optical Cross Connect (OXC), in which conversion into electricity is not performed, will be available, and dynamic path setting/release on the basis of wavelength information will also be available.
An optical switch (hereinafter will be also called the “wavelength-selective switch”) which the present invention relates to is placed in a node in a mesh-type network as shown in FIG. 10 and in a ring-type network as shown in FIG. 11 (reference character 100 designates an wavelength-selective switch in FIG. 10 and FIG. 11), and assigns input wavelengths to arbitrary output ports. Here, in the mesh-type network of FIG. 10, reference characters 200 and 300 designate a multiplexing unit and a demultiplexing unit, respectively. In the ring-type network of FIG. 11, reference characters 201 and 301 designate a multiplexing unit for add light and a demultiplexing unit for drop light, respectively.
This assignment function is equal to a function of a cross bar switch for each wavelength as shown in FIG. 9. That is, the wavelength selective switch module (hereinafter will be simply called the “wavelength selective switch) 100 of FIG. 9 includes, for example, demultiplexing units 101 according to the number of input ports corresponding to the input optical paths (input fibers), 2×2 switches 102, and multiplexing units 103 according to the number of output ports corresponding to the output optical transmission paths (output fibers). The demultiplexing unit 101 separates WDM light, input from the input port, by wavelength (channel), and each wavelength is switched (cross or bar switching) by any of the 2×2 switches 102 according to the setting, and any of the multiplexing units 103 multiplexes the wavelength of light with other wavelengths of light, and outputs the WDM light to the corresponding output port.
For example, as shown in FIG. 9, WDM light at wavelengths of λ2, λ5, and μ6 is input to input port #1, and WDM light at wavelengths of λ1, λ3, λ4, and λ7 are input to input port #2. Light at wavelengths of λ, λ4, and λ6 is switched by the 2×2 switches 102, by cross or bar switching, to be output to the output port #1, and light at the remaining wavelengths of λ2, λ3, λ5, and λ7 are switched to be output to the output port #2. In FIG. 9, reference character 104 designates a gain equalization (optical attenuator) function. In a wavelength-selective optical switch 100 of a spatial join type, the light is collected onto the output fiber at a position appropriately offset from the center of the core, so that the amount of light coupled to the core is varied.
Specifically, as shown in FIG. 12, a known spatial-join-type wavelength-selective switch 100 has a collimator array 111, which forms an input/output optical system, a spectroscope 112, which forms a spectroscopic optical system for separating input WDM light by wavelength, a collective lens 113, which forms a focusing optical system, and a micro mirror array unit 114, which is a switching device. These elements are formed on a substrate 110.
Here, in the collimator array 111, micro lenses (collimate lens; hereinafter will be simply called the “lenses”) are arranged/formed on one side of a glass substrate. On the other side of the substrate, optical fibers are adhered or fused to the positions corresponding to the lenses arranged on the other side so that the optical axes, that is, the centers of the lenses and the centers of the fiber cores, match each other. Light entering a lens from an input optical fiber is converted into collimate light, which is then output to the spectroscope 112. On the other hand, collimate light entering a lens from the spectroscope 112 is focused onto the core of an output fiber. The collimator array 111 of FIG. 12 is a 1-input and 3-output collimator array, which has four optical fibers in total, one input fiber 111-1 which corresponds to an input port, and three output fibers 111-2, 111-3, and 111-4 which correspond to output ports.
The spectroscope 112 reflects incident light in different directions (angles) by wavelength, and it is normally realized by a diffraction grating. FIG. 14 shows a construction of a typical diffraction grating (partially expanded cross sectional view). A diffraction grating is a well-known optical device formed by a glass substrate 120 on which multiple parallel grooves are formed at regular intervals. Utilizing diffraction phenomenon of light, the diffraction grating gives component wavelengths, input at a specific angle (α), different angles of emergence (β). This action makes it possible to separate incident WDM light into its component wavelengths.
The diffraction grating of FIG. 14 has grooves shaped like saw teeth to improve diffraction efficiency, and is called blaze-type diffraction grating. In addition to the diffraction grating of a reflection type of FIG. 14 which reflects incident light, there is another type (transmission-type) of diffraction grating which transmits incident light and realizes a wavelength-separating action equal to the reflective-type diffraction grating. If a transmission-type diffraction grating is employed, collective lens 113 and micro mirror array unit 114 should be arranged after the transmission-type diffraction grating.
The micro mirror array unit 114 functions as a switching device which reflects incident light input from the input fiber 111-1 to any of the output fibers 111-2, 111-3, and 111-4, thereby realizing a port switching function. In the micro mirror array unit 114, micro mirrors (hereinafter will be called “MEMS mirrors”) 140 (see FIG. 15), such as MEMS (Micro Electro Mechanical Systems), are arranged in array form. Specifically, MEMS mirrors 140 are provided, one for each of the wavelengths separated by the spectroscope (diffraction grating) 112. Tilt angles of the MEMS mirrors 140 are variable as shown in FIG. 13(A) and FIG. 13(B), and an output port of each component wavelength is determined (switched) according to the tilt angle.
The collective lens 113 collects a wavelength of light separated by the spectroscope 112 to a specific MEMS mirror 140, and it also collects light reflected by any of the MEMS mirrors 140 to output to the collimator array 111 via the spectroscope 112.
With such a construction, in the wavelength-selective optical switch 100, WDM light input through the input fiber 111-1 of the collimator array 111 is converted into collimate light by the above lens, and enters the spectroscope 112. The light output from the spectroscope 112 at different angles by wavelength enters the collective lens 113, which collects the light to the corresponding MEMS mirrors 140 of the micro mirror array unit 114.
The light input to the MEMS mirrors 140 and reflected thereby goes through a different optical path, and enters any of the output fibers 111-2, 111-3, and 111-4 via the collective lens 113 and the spectroscope 112. When the output fibers 111-2, 111-3, and 111-4 to which the reflected light is to be coupled are changed, the tilt angles of the MEMS mirrors 140 are changed as shown in FIG. 13(A) and FIG. 13(B). In this manner, output switching for each wavelength is realized. In addition, by adjusting the tilt angles of the MEMS mirrors 140, the amount of light coupled to the cores of the output fibers 111-2, 111-3, and 111-4 is controlled, so that an optical attenuator function 104, as already described with reference to FIG. 9, is realized as well as the output switching function.
The following Patent Document 2 discloses a wavelength-selective switch 100 employing MEMS mirrors used in OADM.
A pass band is one of the parameters of performance of the wavelength-selective optical switch 100. As schematically shown in FIG. 15, the pass band is determined as the ratio of the diameter of a beam entering a MEMS mirror 140 to mirror width W. If the pass band is wider, the following advantages are obtained:
(1) loss due to deviation of the center wavelength becomes smaller;
(2) the upper limit of the bit rate supported is improved;
(3) the number of wavelength-selective optical switches 100 connected is increased. In other words, if the pass band is narrow, deterioration of optical power due to deviation of the center wavelength is large, so that good transmission characteristic cannot be maintained.
As already described, in the wavelength-selective optical switch 100, after WDM light is separated into its component wavelengths, a focusing optical system (collective lens 113) collimates each wavelength of light and makes the light hit the corresponding MEMS mirror 140. Thus, a relationship as shown in FIG. 16 is established between the spectroscopic optical system (spectroscope 112) and the distance (mirror pitch) between MEMS mirrors 140. That is, as shown in FIG. 16, if a wavelength interval of WDM light is Δλ, and a separation angle of WDM light by the spectroscopic optical system is β, the wavelength separation ability of the spectroscopic optical system is expressed by dβ/dλ, and a separation angle β is given by β=Δλ·(dβ/dλ). Thus, between the distance L between the spectroscopic optical system (spectroscope 112) and the focusing optical system (collective lens 113) and the mirror pitch P, the following relation is established:
  L  =      P          Δ      ⁢                          ⁢              λ        ·                              ⅆ            β                    /                      ⅆ            λ                              
Hence, for obtaining a wide pass band, the beam diameter of the wavelength which hits each MEMS mirror 140 is set as small as possible in comparison with the mirror width W, and the beam should hit the center of the MEMS mirror 140 as much as possible.
As shown in FIG. 14, if an angle (incident angle) formed between the incident light and the diffraction grating normal is α, and an angle (angle of emergence from the diffraction grating) formed between diffraction light and the diffraction grating normal is β, the following relational expression (1) is established:sin α+sin β=Nmλ  (1)where N is the number of grooves/mm of the diffraction grating, and m is the diffraction order, and λ is a wavelength.
Here, assuming that the incident angle is constant, if both sides of the equation are differentiated, the following equation (2) is obtained.
                                          ⅆ            β                                ⅆ            λ                          =                  Nm                      cos            ⁢                                                  ⁢            β                                              (        2        )            
The both sides of this equation are multiplied by the focal distance fL, and if fL×dβ=dy,
                    dy        =                              f            L                    ×          Δλ          ×                      Nm                          cos              ⁢                                                          ⁢              β                                                          (        3        )            
Here, “dy” is a spatial distance (beam interval) formed by different wavelengths (wavelengths interval Δλ) on the plane of emergence after they pass through the collective lens 113. This equation (3) indicates that the beam interval dy depends on the angle of emergence (diffraction angle β). The appearance of such phenomenon is disclosed in paragraph 0008 through 0010 of the following Patent Document 1.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-294980
[Patent Document 2] Specification of U.S. Pat. No. 5,960,133
Accordingly, considering a case where wavelengths of light with a constant wavelength interval dλ are input to a diffraction grating, and where MEMS mirrors 140 are arranged at constant intervals in the spectral direction, since the beam interval dy output from the diffraction grating depends on the wavelength, if adjustment is performed so that a specific wavelength of beam hits the center of a mirror, beam positions of other wavelengths are deviated from the centers of the corresponding mirrors according to the wavelengths, so that the pass band is deteriorated.
FIG. 17 shows a calculation example where a wavelength range in use is a C band (1528.77 through 1563.05 nm), the wavelength interval (dλ) is 100 GHz, the number of wavelengths is 44, the number (N) of grooves of the diffraction grating is 1200/mm, the diffraction order (m) is 1, the incident angle α is 68°, and a mirror pitch is 250 μm. In FIG. 17, dotted line 400 indicates the position of a MEMS mirror 140 (mm) against the channel number (wavelength), and dotted line 500 indicates the beam position (mm) against the channel number, and reference character 600 indicates the pitch (μm) between adjacent channels against the channel number.
FIG. 17 shows that the center position of a beam at each wavelength output from the diffraction grating gradually deviates from the positions (dotted line 400) of the MEMS mirrors 140 arranged at equal intervals. That is, if the focal distance fL of the collective lens 113 is determined so that the beam interval, after being output from the diffraction grating, between channel number 1 and channel number 2 is 250 μm, the beam interval between channel number 43 and channel number 44 are increased to about 343 μm. In this case, the beam incident position greatly deviates from the center position of the MEMS mirror 140, thereby causing significant deterioration of the pass band.